Nonoxidative Leucine Disposal Research Articles

Effect of oral citrulline supplementation on whole body protein metabolism in adult patients with short bowel syndrome: A pilot, randomized, double-blind, cross-over study

As citrulline is produced by small intestine, plasma citrulline concentration is decreased and may become essential in patients with short bowel syndrome (SBS). In a rat model of SBS, citrulline supplementation enhanced muscle protein synthesis. The aim of the study was to determine whether citrulline impacts whole body protein metabolism in patients with SBS. Nine adults with non-malignant SBS (residual small bowel 90±48cm; mean±SD) who were in near-normal nutritional status without any artificial nutrition, were recruited long after surgery. They received 7-day oral supplementation with citrulline (0.18g/kg/day), or an iso-nitrogenous placebo in a randomized, double-blind, cross-over design with a 13-day wash-out between regimens, and an intravenous 5-h infusion of L-[1-13C]-leucine in the postabsorptive state to assess protein metabolism after each regimen. Plasma citrulline concentration rose 17-fold (25±9 vs. 384±95μmol/L) and plasma arginine 3-fold after oral citrulline supplementation (both p<4×10-6). Supplementation did not alter leucine appearance rate (97±5 vs. 97±5μmolkg-1.h-1; p=0.88), leucine oxidation (14±1 vs. 12±1μmolkg-1.h-1; p=0.22), or non-oxidative leucine disposal (NOLD), an index of whole-body protein synthesis (83±4 vs. 85±5μmolkg-1.h-1; p=0.36), nor insulin or IGF-1 plasma concentrations. In each of the 3 patients with baseline citrulline<20μmol/L, citrulline supplementation increased NOLD. Among the 7 patients with plasma citrulline <30μmol/L, the effect of supplementation on NOLD correlated inversely (r2=0.81) with baseline plasma citrulline concentration. 1) Oral citrulline supplementation enhances citrulline and arginine bioavailability in SBS patients. 2) Oral citrulline supplementation does not have any anabolic effect on whole body protein metabolism in patients with SBS in good nutritional status, in the late phase of intestinal adaptation, and with near-normal baseline citrulline homeostasis. 3) Whether oral citrulline would impact whole body protein anabolism in severely malnourished SBS patients in the early adaptive period, and with baseline plasma citrulline below 20μmol/L, warrants further study. Registered under ClinicalTrials.gov Identifier no. NCT01386034.

Impact of 3-week citrulline supplementation on postprandial protein metabolism in malnourished older patients: The Ciproage randomized controlled trial

Citrulline (CIT), is not extracted by the splanchnic area, can stimulate muscle protein synthesis and could potentially find clinical applications in conditions involving low amino acid (AA) intake, such as in malnourished older subjects. Our purpose was to research the effects of CIT supplementation on protein metabolism in particular on non-oxidative leucine disposal (NOLD, primary endpoint), and splanchnic extraction of amino acids in malnourished older patients. This prospective randomized multicenter study determined whole-body and liver protein synthesis, splanchnic protein metabolism and appendicular skeletal muscle mass (ASMM) in 24 malnourished older patients [80-92 years; 18 women and 6 men] in inpatient rehabilitation units. All received an oral dose of 10g of CIT or an equimolar mixture of six non-essential amino acids (NEAAs), as isonitrogenous placebo, for 3 weeks. NOLD and albumin fractional synthesis rates were not different between the NEAA and CIT groups. Splanchnic extraction of dietary amino acid tended to decrease (p=0.09) in the CIT group (45.2%) compared with the NEAA group (60.3%). Total differences in AA and NEAA area under the curves between fed-state and postabsorptive-state were significantly higher in the CIT than in the NEAA group. There were no significant differences for body mass index, fat mass (FM), lean mass (LM) or ASMM in the whole population except for a tendential decrease in FM for the citrulline group (p=0.089). Compared with Day 1, lean mass and ASMM significantly increased (respectively p=0.016 and p=0.018) at Day 20 in CIT-treated women (mean respective increase of 1.7kg and 1.1kg), and fat mass significantly decreased (p=0.001) at Day 20 in CIT-group women (mean decrease of 1.3kg). Our results demonstrate that CIT supplementation has no effect on whole-body protein synthesis or liver protein synthesis in malnourished older subjects. However, CIT supplementation was associated with a higher systemic AA availability. In the subgroup of women, CIT supplementation increased LM and ASMM, and decreased FM.

Maternal post-absorptive leucine kinetics during late pregnancy in US women with HIV taking antiretroviral therapy: A cross-sectional pilot study

Despite the success of combination antiretroviral therapy (cART) for the prevention of mother to child transmission of HIV, infants exposed to cART in utero frequently are born smaller and have mild cardiac abnormalities. The mechanisms responsible for lower birth weight and cardiac abnormalities in children exposed to cART are unclear but could be related to dysregulation of maternal amino acid metabolism during pregnancy. Previous data in HIV(-) women have shown a relationship between abnormal maternal protein metabolism during pregnancy and low infant birth weight and animal data demonstrate a relationship between altered maternal protein metabolism and increased risk for offspring cardiovascular abnormalities. The objectives of this study were to: characterize post-absorptive maternal leucine kinetics during late pregnancy andexamine the relationships between maternal leucine kinetics and offspring birth weight and cardiac function. Post-absorptive maternal leucine kinetics (evaluated by using stable isotope tracer methodology) in 16 HIV(+) women receiving cART and 14 HIV(-) US women during the 3rd trimester of pregnancy were compared. Relationships between post-absorptive maternal leucine kinetics, cardiac function (echocardiography) and birth weight were statistically examined. Maternal plasma leucine concentration (HIV(-): 82.8 ± 10.7 vs. HIV(+): 72.3 ± 13.5 μM, p=0.06) and leucine oxidation rate (HIV(-): 6.1 ± 1.6 vs. HIV(+): 4.9 ± 1.8 μmol/kgBW/min, p=0.03) were lower in HIV+ women compared to controls. Total leucine turnover rate, non-oxidative leucine disposal rate and post-absorptive maternal glucose and palmitate kinetics did not differ between groups. Left ventricular fractional shortening tended to be lower in children born to HIV(+) compared to controls (HIV(-): 42 ± 1 vs. HIV+: 36 ± 5 %, p=0.08) and associated with lower maternal plasma leucine concentration (r= 0.43, p=0.08). Preliminary results indicate that post-absorptive maternal leucine metabolism during late pregnancy is mildly altered in HIV+ US women taking cART. The clinical significance of maternal leucine metabolism on adverse infant outcomes is unclear and should be further explored in more expansive studies.

Adaptation of in vivo amino acid kinetics facilitates increased amino acid availability for fetal growth in adolescent and adult pregnancies alike.

During pregnancy, adult women with a normal BMI synthesise extra amino acids after an overnight fast by increasing body protein breakdown and decreasing amino acid oxidation. It is not known whether adolescent girls can make these adaptations during pregnancy. The present study aimed to measure and compare the protein, glutamine and alanine kinetics of adult women and adolescent girls at early-, mid- and late-pregnancy. Kinetics were measured in the overnight fasted state using intravenous infusions of 13C-leucine, 15N-glutamine and 15N-alanine in ten adults and twenty adolescents aged 14-17 years in the first and second trimesters (phase 1 study) and infusions of 13C-leucine and 15N2-urea in ten adults and eleven adolescents aged 16-17 years in the first and third trimesters (phase 2 study). In phase 1 study, there were no significant differences between the groups with regard to any of the kinetic parameters measured. In both groups, leucine flux increased (P< 0.05), the percentage of leucine flux oxidised decreased (P< 0.05) and non-oxidative leucine disposal to protein synthesis increased (P< 0.05) from the first to the second trimester. In phase2 study, leucine flux was significantly slower (P< 0.05) in the adult group than in the adolescent group during both trimesters, and whole-body leucine flux and non-oxidative leucine disposal increased significantly in the adolescent group (P< 0.05, respectively) and were higher in the adult group from the first to the third trimester. These results suggest that similar to their adult counterparts after an overnight fast, adolescent girls with a normal BMI provide extra amino acids required for net protein deposition during pregnancy by increasing protein breakdown and decreasing amino acid oxidation.

Growth hormone enhances fat-free mass and glutamine availability in patients with short-bowel syndrome: an ancillary double-blind, randomized crossover study , ,

Benefits of recombinant human growth hormone (rhGH) alone or combined with glutamine in patients with intestinal failure because of short-bowel syndrome remain controversial. We explored effects of rhGH on whole-body protein metabolism in patients with short-bowel syndrome with intestinal failure (SBS-IF) to gain insight into its mechanism of action. Eight stable hyperphagic patients with severe SBS-IF received, in a double-blind, randomized crossover study, low-dose rhGH (0.05 mg · kg⁻¹ · d⁻¹) and a placebo for two 3-wk periods. Leucine and glutamine kinetics under fasting and fed conditions, fat-free mass (FFM), and serum insulin were determined on the final day of each treatment. rhGH increased FFM and nonoxidative leucine disposal (NOLD; an index of protein synthesis) (P < 0.02), whereas FFM and NOLD were correlated in the fed state (r = 0.81, P = 0.015). With rhGH administration, leucine release from protein breakdown (an index of proteolysis) decreased in the fed compared with fasting states (P = 0.012), which was not observed with the placebo. However, the fast-to-fed difference in leucine release from protein breakdown was not significantly different between rhGH and placebo (P = 0.093). With rhGH, the intestinal absorption of leucine and glutamine increased (P = 0.036) and correlated with serum insulin (r = 0.91, P = 0.002). rhGH increased glutamine de novo synthesis (P < 0.02) and plasma concentrations (P < 0.03) in both fasting and fed states. In SBS-IF patients, feeding fails to decrease proteolysis in contrast to what is physiologically observed in healthy subjects. rhGH enhances FFM through the stimulation of protein synthesis and might decrease proteolysis in response to feeding. Improvements in de novo synthesis and intestinal absorption increase glutamine availability over the physiologic range, suggesting that beneficial effects of rhGH in hyperphagic patients might be achieved without glutamine supplementation.

Resting energy expenditure and protein turnover are increased in patients with severe chronic obstructive pulmonary disease

The mechanisms leading to weight loss in patients with chronic obstructive pulmonary disease (COPD) are poorly understood. Changes in protein metabolism and systemic inflammation may contribute to increased resting energy expenditure (REE) in COPD, leading to an energy imbalance and loss of fat and fat-free mass. The objective of this study was to determine first whether REE was increased in patients with COPD and, second, whether this was associated with increased protein turnover and/or systemic inflammation. Resting energy expenditure was determined using indirect calorimetry in 14 stable outpatients with severe COPD (7 with low and 7 with preserved body mass indices) and 7 healthy controls. Endogenous leucine flux, leucine oxidation, and nonoxidative disposal, indices of whole-body protein breakdown, catabolism, and synthesis, were measured using intravenous infusions of (13)C-bicarbonate and 1-(13)C-leucine. Total body water, from which fat-free mass and fat mass were calculated, was determined using an intravenous bolus of deuterated water. Plasma markers of systemic inflammation were also measured. As a group, subjects with COPD had increased REE adjusted for fat-free mass (P < .001) and faster rates of endogenous leucine flux (P = .006) and nonoxidative leucine disposal (P = .002) compared with controls. There was a significant correlation between REE and both endogenous leucine flux (P = .02) and nonoxidative leucine disposal (P = .008). Plasma concentrations of the inflammatory markers C-reactive protein and interleukin-6 were not different between COPD subjects and controls. Increased rates of protein turnover are associated with increased REE and loss of fat-free mass in COPD.

Leucine supplementation during endurance exercise enhances post‐exercise muscle protein synthesis

The benefit of leucine-enriched amino acid supplementation on muscle protein synthesis following resistance exercise has been well established. However, the effects after sustained aerobic-type exercise are not well described and may differ, given that muscle contractile forces generated are normally lower. This study examined how essential amino acid (EAA) supplementation, of varying leucine content, affected mixed muscle protein synthesis (MPS), whole body leucine oxidation (Ox), and non-oxidative leucine disposal (NOLD) following sustained aerobic-type exercise. Ten adults completed two separate 60 min bouts of cycle ergometry (60% VO2peak), during which isonitrogenous (10 g EAA) drinks containing leucine-enriched EAA (L-EAA; 34% leucine) or EAA (18% leucine) were consumed. MPS and leucine kinetics were determined using primed, continuous infusions of [2H5]-phenylalanine and [13C]-leucine. Plasma leucine levels were higher (P < 0.05) post-exercise for L-EAA relative to EAA for up to 3.5 h into recovery. Post-exercise MPS was higher (P < 0.05) for L-EAA compared to EAA (0.078 ± 0.01 vs. 0.056 ± 0.01 %/h). Post-exercise leucine Ox was 65% higher (P < 0.05) and NOLD was 19% lower (P < 0.05) for L-EAA than for EAA. Leucine supplementation during sustained steady-state exercise appears to have similar beneficial effects on MPS as during resistance-type exercise.

Impaired Leucine Oxidation During Hyperglycemia After Eccentric Exercise in Older Men

Aging reduces insulin sensitivity in healthy sedentary adults, which may alter protein metabolism during periods of metabolic and physiologic stress. We determined the effects of age, and metabolic (hyperglycemia) and physiological [eccentric exercise (ECE)] stress, on whole-body amino acid turnover. [1-13C]-Leucine kinetics was measured in 8 healthy young (YM, 22±2 y; BMI 23±5 kg/m2) and 8 healthy older (OM, 66±6 y; BMI 25±3 kg/m2) men during fasting (F) and 3-hr hyperglycemic-hyperinsulinemic clamps (HG) that were performed before (CON) and 48 hours after ECE. Leucine oxidation (Lox) decreased (p<0.05) in YM during CON-HG, ECE-F and ECE-HG (CON-F 28.3±3.9; CON-HG 25.9±4.0; ECE-F 24.8±4.1; ECE-HG 21.4±3.2 μmol·kg FFM−1·h−1). In contrast, OM failed to suppress Lox from CON-F conditions (CON-F 26.0±3.9; CON-HG 26.8±3.7; ECE-F 25.1±5.7; ECE-HG 26.3±7.0 μmol·kg FFM−1·h−1; p>0.05). Insulin sensitivity was inversely correlated with Lox (r = −0.53; p<0.05) and was lower in OM (YM 7.1±0.6; OM 4.5±0.3 mg·kg FFM−1·min−1; p<0.05). Nonoxidative leucine disposal and leucine flux were not different between YM and OM (p>0.05). In conclusion, aging leads to an impaired capacity to adjust protein metabolism in response to metabolic or physiological stress, which is coupled with decreased insulin sensitivity. Supported by NIH grants T32DK007319 AG12834, AG15385, AG11811, RR000954, DK020579, DK056341

Protein consumption following aerobic exercise increases whole-body protein turnover in older adults

Research measuring whole-body protein turnover (WBPT) after both exercise and nutrition has generally focused on resistance exercise; however, there is a paucity of data regarding the effect of postaerobic exercise nutrition, especially in older adults. It is not known if postexercise protein feeding has a beneficial effect on protein turnover after low- to moderate-intensity exercise. We investigated whether consuming protein plus carbohydrate (PRO) immediately after an acute bout of aerobic exercise has an additive effect over carbohydrate alone (CHO) on WBPT in older individuals. Twelve healthy older adults (age, 59 ± 4 years) were studied on 2 separate occasions after 1 h of exercise at approximately 50% of maximal rate of oxygen uptake, followed by 4 h of recovery. Immediately following exercise, subjects ingested a CHO (60 g) or an isocaloric PRO beverage (40 g carbohydrate, 20 g whey protein). Whole-body protein metabolism was determined using [1-13C]leucine infusion (60 mg prime; 75 mgh(-1) continuous), and sampling blood and expired breath. Rates of whole-body leucine appearance and oxidation, and nonoxidative leucine disposal during the third and fourth hours of postexercise recovery were higher in the PRO group (2.51 ± 0.55, 0.78 ± 0.37, and 1.71 ± 0.44 micromol kg(-1)·min(-1), respectively) than in the CHO group (1.81 ± 0.27, 0.33 ± 0.14, and 1.47 ± 0.25 micromol kg(-1)·min-1, respectively; p = 0.001). Our results indicate that consumption of a PRO beverage after aerobic exercise increased WBPT to a greater extent than a CHO beverage.

Effects of Adiposity and 30 Days of Caloric Restriction Upon Protein Metabolism in Moderately vs. Severely Obese Women

Protein metabolism adapts during caloric restriction (CR) to minimize protein loss, and it is unclear whether greater fat stores favorably affect this response. We sought to determine whether protein metabolism is related to degree of obesity and whether the response to CR is impacted by pre-CR adiposity level. Whole body protein metabolism was studied in 12 obese women over a wide range of BMI (30-53 kg/m(2)) as inpatients using [1-(13)C]leucine as a tracer following 5 days of a weight-maintaining diet and then after 30 days of CR (1,400 kcal deficit with maintained protein intake). When expressed as total rates, per body weight (BW) or per fat-free mass (FFM), leucine rate of appearance (Ra), and nonoxidative leucine disposal (NOLD) were significantly higher in the individuals with a greater degree of obesity (P < 0.05). Leucine oxidation (Rox) was also higher in more highly obese women when expressed as a total rate (P < 0.05) but not if expressed per BW or FFM. CR reduced BW, FFM, and fat mass (P < 0.001), and declines were relatively similar between individuals. CR reduced Ra (P < 0.001), NOLD (P < 0.01), and Rox (P < 0.05), and the relative decline was not affected by differences in fat mass. CR-induced declines were significant even when Ra and NOLD were normalized to BW or FFM. We conclude that fat mass, like FFM, is a key determinant of protein turnover. However, during CR, higher fat mass does not favorably alter the response of protein metabolism and does not mitigate the loss of FFM.

The Effects of Growth Hormone and/or Testosterone on Whole Body Protein Kinetics and Skeletal Muscle Gene Expression in Healthy Elderly Men: A Randomized Controlled Trial

Alterations of protein turnover may contribute to the progressive decline of muscle mass with aging. Our objective was to examine the effects of near-physiological recombinant human GH and/or testosterone (T) administration to older men on whole body protein kinetics and muscle gene expression. A 6-month randomized, double-blind, placebo-controlled trial in 21 healthy elderly men aged 65-75 yr, was performed. Participants were randomized to receive placebo GH and placebo T, rhGH and placebo T (GH), T and placebo GH (T), or rhGH and T (GHT). The leucine rate of appearance (index of proteolysis), nonoxidative leucine disposal rate (an index of protein synthesis), and leucine oxidation rate were measured with an infusion of l-[1-(13)C] leucine. Muscle biopsies for the measurement of gene expression were performed. Body composition and aerobic capacity (maximal oxygen capacity) were measured. Serum IGF-I levels increased significantly with GH and GHT (P < 0.001) compared with placebo. T increased significantly only in the T group (P = 0.028). Leucine rate of appearance and nonoxidative leucine disposal rate increased with GH (P = 0.015, P = 0.019) and GHT (P = 0.017, P = 0.02), but leucine oxidation did not change significantly in any treatment group. Midthigh muscle mass and maximal oxygen capacity increased (P < 0.04) with GHT only. Expression of muscle function genes did not change significantly, but the within-group comparisons revealed a significant increase of androgen receptor expression in the GHT group (P = 0.001). This study showed that 6-month treatment with low-dose GH alone or with T in healthy elderly men produces comparable increments in whole body protein turnover and protein synthesis.

The Effects of Nutritional Supplementation Throughout an Endurance Run on Leucine Kinetics during Recovery

This study determined the effect of nutritional supplementation throughout endurance exercise on whole-body leucine kinetics (leucine rate of appearance [Ra], oxidation [Ox], and nonoxidative leucine disposal [NOLD]) during recovery. Five trained men underwent a 2-h run at 65% VO(2max), during which a carbohydrate (CHO), mixed protein-carbohydrate (milk), or placebo (PLA) drink was consumed. Leucine kinetics were assessed during recovery using a primed, continuous infusion of 1-13C leucine. Leucine Ra and NOLD were lower for milk than for PLA. Ox was higher after milk-supplemented exercise than after CHO or PLA. Although consuming milk during the run affected whole-body leucine kinetics, the benefits of such a practice for athletes remain unclear. Additional studies are needed to determine whether protein supplementation during exercise can optimize protein utilization during recovery.

A Randomized Controlled Trial of the Anticatabolic Effect of Epidural Analgesia and Hypocaloric Glucose

The goal of the present study was to investigate whether epidural analgesia exerts a protein-sparing effect after colorectal surgery in the presence of hypocaloric glucose supply initiated with surgical skin incision. We randomly allocated 10 patients to receive general anesthesia combined with epidural anesthesia with bupivacaine, followed by epidural analgesia using bupivacaine/fentanyl, and 10 patients to receive general anesthesia, followed by patient-controlled analgesia with intravenous morphine. All patients received a 48-hour infusion of glucose 10% from surgical skin incision until the second day after surgery. The glucose infusion rate provided 50% of the patient's resting energy expenditure. Kinetics of protein and glucose metabolism were assessed by a stable-isotope tracer technique (L-[1-(13)C]leucine and [6,6-(2)H(2)]glucose). The rate of appearance of leucine increased in the intravenous-analgesia group (112 +/- 29 to 130 +/- 25 micromol/kg/h) 2 days after surgery, and this increase was more pronounced than in the epidural analgesia group (preoperative 120 +/- 24, postoperative 123 +/- 22 micromol/kg/h, P < .05). Leucine oxidation rate increased in the intravenous analgesia group from 17 +/- 8 to 23 +/- 8 micromol/kg/h and in the epidural group from 17 +/- 6 to 19 +/- 7 micromol/kg/h without the difference between the groups reaching statistical significance (P = .067). Nonoxidative leucine disposal remained unaltered in both groups. No differences in glucose metabolism were seen between the groups. Epidural analgesia inhibits the increase in whole-body protein breakdown in patients receiving perioperative hypocaloric glucose infusion initiated with surgical skin incision. However, oxidative protein loss, protein synthesis, and glucose metabolism are not affected by epidural analgesia.

Resistance Training Reduces Fasted- and Fed-State Leucine Turnover and Increases Dietary Nitrogen Retention in Previously Untrained Young Men1

We aimed to determine the impact of intense resistance training, designed to increase lean body mass (LBM), on both fasted and fed whole body protein kinetics in untrained young men. Twelve healthy males (22 ± 2 y of age; BMI, 24.3 ± 2.4 kg/m2) participated in a 12-wk (5-d/wk) resistance training program. Before and after training, a primed constant infusion of [1-13C]leucine was used to measure whole body leucine turnover, protein breakdown, and nonoxidative leucine disposal in the fasted and fed states. Participants were studied during 5-d controlled diet periods that provided a moderate protein intake [1.4 g/(kg body wt · d)]. We estimated protein turnover and nitrogen balance. Training increased LBM (61.6 ± 6.9 vs. 64.8 ± 6.7 kg, P < 0.05). After training, whole body leucine turnover was reduced (P < 0.01) in both fasted (167 ± 18 vs. 152 ± 17) and fed (197 ± 23 vs. 178 ± 21) states [all values μmol/(kg LBM · h)]. Training-induced decreases (P < 0.01) in protein breakdown occurred in the fasted (165 ± 18 vs. 144 ± 17) and fed (111 ± 23 vs. 93 ± 20) states. Following training, nonoxidative leucine disposal was similarly reduced (P < 0.01) in the fasted (144 ± 18 vs. 126 ± 18) and fed (151 ± 20 vs. 133 ± 19) states. Nitrogen balance was more positive after training (13.7 ± 8.1 vs. 33.4 ± 12.5 g/(kg LBM · d), P < 0.01) indicating an increased retention of dietary nitrogen. Intense resistance training alters whole body protein kinetics in novice weightlifters regardless of feeding status. The increase in nitrogen balance after training demonstrates a more efficient utilization of dietary nitrogen, suggesting that protein requirements for novice weightlifters are not elevated.

Postexercise Whole-Body Protein Turnover Response to Three Levels of Protein Intake

This investigation examined the effect of variations in protein intake on whole-body protein turnover (WBPTO) after exercise in endurance-trained males. Five male runners (21.3 +/- 0.3 yr, 179 +/- 2 cm, 70.6 +/- 0.1 kg, 8.7 +/- 0.4% body fat, 70.6 +/- 0.1 VO2peak) participated in a randomized, crossover-design diet intervention, where they consumed either a low- (0.8 g.kg(-1); LP), moderate- (1.8 g.kg(-1); MP), or high-protein (3.6 g.kg(-1); HP) diet for 4 wk. WBPTO (Ra, leucine rate of appearance; NOLD, nonoxidative leucine disposal; and Ox, leucine oxidation) were assessed after a 75-min run at 70% VO2peak after each diet-intervention period. Leucine Ra (indicator of protein breakdown) and leucine Ox were greater on the HP diet than on the LP diet (Ra, 123.4 +/- 6.9 vs 97.9 +/- 6.0 micromol.kg(-1).h(-1); Ox, 23.9 +/- 0.5 vs 17.0 +/- 0.8 micromol.kg(-1).h(-1), P < 0.05). No differences were noted in NOLD (an indicator of protein synthesis) across diets. Plasma branched chain amino acids (BCAA) at rest were greater for MP and HP than for LP, and nonessential amino acids (NEAA) were greater for LP than MP at rest and greater than MP and HP after exercise. Findings from this study show that variations in protein intake can alter plasma amino acid levels and modulate rates of WBPTO after exercise. Additionally, a lower protein intake was associated with decreased rates of WBPTO after exercise.

Protein Metabolism in Acromegaly: Differential Effects of Short- and Long-Term Treatment

GH acutely increases body protein by stimulating protein synthesis and reducing protein oxidation. The objective of the study was to determine whether these changes in protein metabolism are sustained in long-term GH excess and reversed by correction. We conducted a cross-sectional study in 16 acromegalic and 18 normal subjects and a longitudinal study in which acromegalic subjects were studied before and after short-term (n=8) or long-term (n=10) treatment. The study was conducted at a clinical research center. Whole-body rates of leucine appearance (leucine Ra; an index of protein breakdown), leucine oxidation, and nonoxidative leucine disposal (NOLD; an index of protein synthesis) estimated using infusion of 1-[13C] leucine were measured. Leucine Ra and NOLD were greater (P<0.01) in acromegalic compared with normal subjects, whereas leucine oxidation did not differ. Leucine oxidation increased significantly (P<0.05) after short-term treatment but returned to baseline after long-term treatment. Both leucine Ra and NOLD decreased significantly (P<0.05) after short- and long-term treatment. Adjustment for body composition did not affect results. In acromegalic subjects, protein breakdown and synthesis are increased, whereas protein oxidation does not differ from normal subjects. Protein oxidation increases transiently, whereas protein breakdown and synthesis are stably reduced after treatment. Because protein oxidation represents irreversible loss, we conclude that the normal state of protein oxidation found in acromegaly and after long-term treatment represents metabolic adaptation, which maintains protein mass at a steady state after stable changes in GH status.

Level of dietary protein impacts whole body protein turnover in trained males at rest

The current investigation examined the effect of variations in protein intake on Whole body protein turnover (WBPTO) at rest in endurance-trained males. Whole body protein turnover is influenced by both diet and exercise. Whether endurance athletes require more protein than the non-exerciser remains equivocal. Five male runners (21.3 ± 0.3 years, 179 ± 2 cm, 70.6 ± 0.1 kg, 8.7% ± 0.4% body fat, 70.6 ± 0.1 V˙ o 2max) participated in a randomized, crossover design diet intervention where they consumed either a low-protein (LP; 0.8 g/kg), moderate-protein (MP; 1.8 g/kg), or high-protein (HP; 3.6 g/kg) diet for 3 weeks. Whole body protein turnover (Ra, leucine rate of appearance; NOLD, nonoxidative leucine disposal; and Ox, leucine oxidation), nitrogen balance, and substrate oxidation were assessed at rest following each dietary intervention period. The HP diet increased leucine Ra (indicator of protein breakdown; 136.7 ± 9.3, 129.1 ± 7.4, and 107.8 ± 3.1 μmol/[kg · h] for HP, MP, and LP diets, respectively) and leucine Ox (31.0 ± 3.6, 26.2 ± 4.3, and 18.3 ± 0.6 μmol/[kg · h] for HP, MP, and LP diets, respectively) compared with LP diet ( P < .05). No differences were noted in nonoxidative leucine disposal (an indicator of protein synthesis) across diets. Nitrogen balance was greater for HP diet than for MP and LP diets (10.2 ± 0.7, 1.8 ± 0.6, and −0.3 ± 0.5 for HP, MP, and LP diets, respectively). Protein oxidation increased with increasing protein intake (54% ± 6%, 25% ± 1%, and 14% ± 2% for HP, MP, and LP diets, respectively). Findings from this study show that variations in protein intake can modulate WBPTO and that protein intake approximating the current recommended dietary allowance was not sufficient to achieve nitrogen balance in the endurance-trained males in this investigation. Our results suggest that a protein intake of 1.2 g/kg or 10% of total energy intake is needed to achieve a positive nitrogen balance. This is not a concern for most endurance athletes who routinely consume protein at or above this level.

Effect of increasing dietary protein content on muscle protein metabolism in healthy young and older people

Aging is associated with decreasing muscle mass and protein synthesis. Amino acid administration is reported to increase muscle protein synthesis and balance. We determined whether increasing dietary protein content from adequate (1.5g/kgFFM/d, AP) to high (3g/kgFFM/d, HP) had beneficial or adverse effects on whole body and muscle protein metabolism, renal function and insulin sensitivity in healthy young (24±1y) and older (70±2y) men and women. Subjects were randomized to a crossover design of 10 d of AP or HP protein diets, without changing caloric intake. Measurements of postabsoptive whole body leucine appearance rate (protein breakdown), oxidation, and non-oxidative leucine disposal (protein synthesis) rates were performed using L-[1-13C]-leucine and fractional synthesis rate (FSR) of mixed muscle proteins was done using [15N]-lysine as tracers. Glomerular filtration rate (GFR) and insulin sensitivity were determined by iodothalamate and intravenous glucose tolerance test, respectively. HP diet increased protein breakdown in both young and old (p<0.05) and increased leucine oxidation in both age groups with greater increase in young (p<0.005). WBPS, muscle protein FSR and insulin sensitivity were unaffected by diet. GFR was lower (p<0.01) in older than in young during both diets. HP diet increased GFR in young (p<0.01) but decreased GFR in older people (p<0.01). In conclusion, increasing dietary protein content had no detectable beneficial effects on muscle or whole body protein metabolism in healthy older people and may cause deleterious effects on renal function. Support: NIH R01AG09531.

Whole body leucine flux in HIV-infected patients treated with or without protease inhibitors

The present study was carried out to assess the effects of protease inhibitor (PI) therapy on basal whole body protein metabolism and its response to acute amino acid-glucose infusion in 14 human immunodeficiency virus (HIV)-infected patients. Patients treated with PIs (PI+, 7 patients) or without PIs (PI-, 7 patients) were studied after an overnight fast during a 180-min basal period followed by a 140-min period of amino acid-glucose infusion. Protein metabolism was investigated by a primed constant infusion of l-[1-(13)C]leucine. Dual-energy X-ray absorptiometry for determination of fat-free mass (FFM) and body fat mass measured body composition. In the postabsorptive state, whole body leucine balance was 2.5 times (P < 0.05) less negative in the PI+ than in the PI- group. In HIV-infected patients treated with PIs, the oxidative leucine disposal during an acute amino acid-glucose infusion was lower (0.58 +/- 0.09 vs. 0.81 +/- 0.07 micromol x kg FFM(-1) x min(-1) using plasma [(13)C]leucine enrichment, P = 0.06; or 0.70 +/- 0.10 vs. 0.99 +/- 0.08 micromol x kg FFM(-1) x min(-1) using plasma [(13)C]ketoisocaproic acid enrichment, P = 0.04 in PI+ and PI- groups, respectively) than in patients treated without PIs. Consequently, whole body nonoxidative leucine disposal (an index of protein synthesis) and leucine balance (0.50 +/- 0.10 vs. 0.18 +/- 0.06 micromol x kg FFM x (-1) x min(-1) in PI+ and PI- groups respectively, P < 0.05) were significantly improved during amino acid-glucose infusion in patients treated with PIs. However, whereas the response of whole body protein anabolism to an amino acid-glucose infusion was increased in HIV-infected patients treated with PIs, any improvement in lean body mass was detected.

Intraoperative protein sparing with glucose

We examined the hypothesis that glucose infusion inhibits amino acid oxidation during colorectal surgery. We randomly allocated 14 patients to receive intravenous glucose at 2 mg x kg(-1) x min(-1) (glucose group) starting with the surgical incision or an equivalent amount of normal saline 0.9% (control group). The primary endpoint was whole body leucine oxidation; secondary endpoints were leucine rate of appearance and nonoxidative leucine disposal as determined by a stable isotope tracer technique (L-[1-(13)C]leucine). Circulating concentrations of glucose, lactate, insulin, glucagon, and cortisol were measured before and after 2 h of surgery. Leucine rate of appearance, an estimate of protein breakdown, and nonoxidative leucine disposal, an estimate of protein synthesis, decreased in both groups during surgery (P < 0.05). Leucine oxidation intraoperatively decreased from 13 +/- 3 to 4 +/- 3 micromol x kg(-1) x h(-1) in the glucose group (P < 0.05 vs. control group) whereas it remained unchanged in the control group. Hyperglycemia during surgery was more pronounced in patients receiving glucose (9.7 +/- 0.5 mmol/l, P < 0.05 vs. control group) than in patients receiving normal saline (7.1 +/- 1.0 mmol/l). The administration of glucose caused an increase in the circulating concentration of insulin (P < 0.05) resulting in a lower glucagon/insulin quotient than in the control group (P < 0.05). Intraoperative plasma cortisol concentrations increased in both groups (P < 0.05), whereas plasma concentrations of lactate and glucagon did not change. The provision of small amounts of glucose was associated with a decrease in amino acid oxidation during colorectal surgery.